Porous membranes based on fluoropolymers are known in the art which are endowed with outstanding chemical, thermal and mechanical resistance.
Among frequently used methods for producing porous membranes of vinylidene fluoride (VDF) polymers, methods wherein the surface of the membrane is coated or crosslinked with (meth)acrylic monomers are known in the art.
For instance, U.S. Pat. No. 4,774,132 (PALL CORPORATION) 27 Sep. 1988 discloses a method for manufacturing porous membranes based on vinylidene fluoride polymers obtained by graft-polymerizing onto a polyvinylidene fluoride substrate a vinylic monomer. Preferred vinylic monomers include, notably, acrylic acid, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate.
Also, membranes of VDF polymers have been proposed in the past which do not require any further grafting/coating step; thus, EP 1621573 A (KUREHA CORPORATION) 1 Feb. 2006 discloses porous membranes prepared by a composition comprising a vinylidene fluoride (VDF) copolymer with 0.01-10% by moles of a monomer, said monomer having at least one group selected from an epoxy, a hydroxyl, a carboxyl, an ester, an amide and an acid anhydride group, a plasticizer and a suitable solvent. The composition so obtained is melt-extruded into a film, the film is cooled to be solidified, then subjected to extraction of the plasticizer and further stretched.
Further, WO 2008/129041 (SOLVAY SOLEXIS S.P.A.) 30 Oct. 2008 discloses porous membranes of a linear semi-crystalline VDF copolymer with 0.05-10% by moles of a (meth)acrylic monomer, wherein recurring units derived from (meth)acrylic monomers are randomly distributed throughout the whole vinylidene fluoride backbone. Commonly used processes for the manufacture of said porous membranes of VDF copolymers typically include at least one step including one of irradiation, film expansion, template leaching, solution precipitation techniques.
Membranes based on ethylene/chlorotrifluoroethylene (ECTFE) or ethylene/tetrafluoroethylene (ETFE) polymers have been also disclosed in the past.
Thus, U.S. Pat. No. 4,702,836 (ASAHI KASEI) 27 Oct. 1987 discloses porous membranes made of a fluorinated resin selected from ECTFE, ETFE and/or polychlorotrifluoroethylene (PCTFE). These membranes are obtained by melt-moulding a composition comprising the fluorinated resin, an inorganic fine powder material and a mixture of a chlorotrifluoroethylene oligomer and certain organic heat resistant substances, removing by extraction with a suitable solvent the chlorotrifluoroethylene oligomer and the organic heat resistant substance from the molded product obtained and further removing by extraction therefrom with a suitable solvent the inorganic fine powder material.
Also, WO 03/068374 (US FILTER WASTEWATER GROUP) 21 Aug. 2003 discloses porous polymeric ultrafiltration or microfiltration membranes including ECTFE and methods of production thereof wherein a mixture of ECTFE with a solvent system is heated and then rapidly cooled so that a non-equilibrium liquid-liquid phase separation occurs to form a continuous polymer rich phase and a continuous polymer lean phase, then removing the polymer lean phase from the solid polymeric material.
Finally, EP 1236503 A (AUSIMONT S.P.A.) 4 Sep. 2002 discloses porous membranes of a semi-crystalline fluoropolymer comprising (a) ethylene, (b) chlorotrifluoroethylene or tetrafluoroethylene, and (c) an hydrogenated monomer, which can be notably an acrylic monomer, such as, inter alia, acrylic acid, hydroxyethylacrylate, hydroxypropylacrylate, (hydroxy)ethylhexylacrylate. These membranes are prepared by processing a solution of said fluoropolymers with suitable plasticizers and then by extracting therefrom the plasticizer by dipping the membrane into a suitable solvent.
Nevertheless, due to certain environmental concerns, efforts are now devoted to find out methods wherein porous membranes of fluoropolymers are manufactured using alternative solvents having a more favourable toxicological profile.
There is thus still a need in the art for a process for manufacturing porous membranes based on fluoropolymers which advantageously enables obtaining porous membranes in a convenient way, while also avoiding use of polluting solvents and thus eliminating cost, safety and environmental concerns related to handling of large volume of said solvents.